(a) Fields of the Invention
The present invention relates to nitride semiconductor structures which are employed for optical devices and the like such as light emitting diodes and semiconductor laser diodes, and to fabrication methods of such structures.
(b) Description of Related Art
Compounds expected to be applied to visible light emitting devices, high-temperature operable electronic devices, or the like include nitride semiconductors indicated by AlGaInN and containing Al, Ga, In or the like as a group III element and N as a group V element. Semiconductor devices employing AlGaInN are being put to practical use in the field of a blue or green light emitting diode and a blue-violet laser diode.
In fabricating a light emitting element using this nitride semiconductor, growing a crystal of a nitride semiconductor thin film by a metal-organic chemical vapor deposition (MOCVD) method is the mainstream. This technique is carried out in the following manner. A reaction tube with a substrate of, for example, sapphire, SiC, GaN, or Si placed therewithin is supplied with trimethyl gallium (abbreviated hereinafter as “TMG”), trimethyl aluminum (abbreviated hereinafter as “TMA”), trimethyl indium (abbreviated hereinafter as “TMI”) or the like as a group III material gas, and also supplied with ammonia, hydrazine, or the like as a group V material gas. While the temperature of the substrate is kept at a high temperature of about 600 to 1200° C., an n-type layer, a light emitting layer, and a p-type layer are grown on the substrate to stack nitride semiconductor layers. The growth of the n-type layer is conducted while monosilane (SiH4) or the like as an n-type impurity material gas is flowed with a group III material gas, and the growth of the p-type layer is conducted while cyclopentadienyl magnesium (Cp2Mg) or the like as a p-type impurity material gas is flowed with a group III material gas.
After this growth step, the surfaces of the n-type layer and the p-type layer are formed with an n-type electrode and a p-type electrode, respectively, and the resulting substrate is separated in chip shapes to fabricate light emitting elements.
As the material for the light emitting layer, use is made of InGaN in which In composition is adjusted to have a desired light-emission wavelength. This light emitting layer is sandwiched by cladding layers with a larger band gap energy than the light emitting layer to construct a double heterostructure, or this light emitting layer is made of a thin film layer capable of producing a quantum size effect to construct a quantum well structure. These two structures have been studied actively in recent years.
The quantum well structure is constructed in the manner in which a layer with a smaller band gap energy (a well layer) is sandwiched by barrier layers with a large band gap energy. In the case of using the quantum well structure for an active layer, use is made of a single quantum well structure (SQW) having one well layer or a multiple quantum well structure (MQW) in which the well layer and the barrier layer are alternately formed. Of the two structures, the MQW is conventionally fabricated by any one of three related arts that will be shown below.
A first related art is the method disclosed in Japanese Unexamined Patent Publication No. H10-12922. In this method, using an MOCVD apparatus, a MQW structure is constructed by repeatedly forming a quantum well structure composed of InGaN as a well layer and AlGaN as a barrier layer.
A second related art is the method disclosed in Japanese Patent No. 3304787. In this method, adjustment of thicknesses of the barrier layers uniformizes the thicknesses of the barrier layers after the growth of the cladding layer to prevent wavelength shift of emitted light.
A third related art is the method disclosed in Japanese Unexamined Patent Publication No. 2002-43618. This method is characterized in that GaN as part of a barrier layer is formed with the temperature elevated to the growth temperature of the barrier layer, thereby preventing degradation of a well layer.